Coating compositions and processes

ABSTRACT

An undercoat composition comprises a film-forming synthetic resin and contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal by a stripping composition of a top coat paint which is subsequently applied over the undercoat. A top coat composition for applying as the outer coating on a substrate comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin and contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal of the cured top coat by a stripping composition from a cured undercoat of similar composition which does not contain the said acidic aromatic polar organic compound.

FIELD OF THE INVENTION

This invention relates to the application to a substrate of a coatingsystem in which a cured primer coating is ,overcoated with a top coatwhich is intended to be strippable, that is to say removable from theprimed substrate by a paint stripping composition without removal of theprimer from the substrate. Such coating systems are widely used onaircraft and may also be used on other substrates, for examplecommercial vehicles, cars, yachts or railway vehicles.

The cured primer and top coat must resist water and certain organicsolvents such as aviation fuel and preferably also resist hydraulicfluids based on phosphate esters such as tricresyl phosphate ortri-n-butyl phosphate, e.g. that sold under the trade mark “Skydrol”.Each coating is generally cured by the reaction of components which havebeen packaged separately and are mixed at or shortly before application.When an aircraft is to be repainted it is usual to remove at least thetop coat using a paint stripper before repainting. The preferredstripping compositions used nowadays are based on aqueous benzyl alcoholwith additives such as acidic (formic or oxalic acid) or alkaline (to pH11 or 12) or neutral additives, although some halogenated solvents suchas methylene chloride with phenol are still used. In general, the topcoat adheres to the primer so strongly that the top coat cannot bestripped without removing or at least damaging the primer. Aircraftowners and operators would prefer a paint system in which the primerremained on the aircraft to prevent the stripping composition contactingany resins, sealants or adhesives used in the aircraft construction orthe metallic substrate and to avoid repeating the pre-treatment andprimer coating of the aircraft.

BACKGROUND ART

GB-A-1511935 describes a strippable coating system comprising anon-crosslinked polyamide intermediate coating between the primer andthe top coat. This at least partially resists the solvent used to removethe top coat and can itself easily be removed by alcohol-based solventsto reveal the primer. DE-A-2528943 describes a linear non-crosslinkedelastic polyurethane coating used between crosslinked polyurethaneprimer and top coat layers. EP-A-147984 describes a barrier intermediatecoating comprising a film-forming addition polymer comprising units of avinyl aromatic monomer and units of a monomer containing a hydroxy alkylgroup in an amount to provide a hydroxyl content of the addition polymerin the range 0.5 to 5 per cent by weight reacted with a polyisocyanatecontaining at least 2 isocyanate groups per molecule.

WO-A-98/13148 describes a process for overcoating a substrate having acured primer coating with a curable top coat which when cured isswellable by a stripping composition, in which process a non-volatilepolar material having high affinity for the primer surface is applied tothe primed substrate from an aqueous liquid vehicle before coating withthe top coat, whereby the said polar material facilitates the removal ofthe top coat from the primer by the stripping composition.

EP-A-14597 describes a paint undercoat in which the film-forming resinhas an acid value of from 10 to 45 mg KOH/g. Paint films including suchan undercoat are stripped by means of a paint stripper having a pH offrom 8 to 14 which attacks the undercoat paint film.

DISCLOSURE OF THE INVENTION

We have found according to the present invention that many of thenon-volatile polar materials used in the process of WO-A-98/13148 canalso facilitate selective stripping of a top coat from an undercoat whenthey are included in one of the coating compositions.

Thus, according to one aspect of the invention an undercoat compositionwhich comprises a film-forming synthetic resin is characterised in thatthe undercoat composition contains a non-volatile acidic aromatic polarorganic compound in free acid or salt form, which compound facilitatesthe removal by a stripping composition of a top coat paint which issubsequently applied over the undercoat. Preferably, the synthetic resincontains functional groups whereby it is curable and the undercoatcomposition also contains a curing agent for crosslinking the syntheticresin.

A coating process according to the invention comprises applying to asubstrate an undercoat composition (preferably a curable undercoatcomposition which is then cured), applying a curable top coatcomposition over the undercoat and curing the top coat, the cured topcoat being swellable by a stripping composition, and it is characterisedin that the undercoat composition contains a non-volatile acidicaromatic polar organic compound in free acid or salt form, whichcompound facilitates the removal of the top coat from the undercoat bythe stripping composition.

The undercoat composition can for example be an anticorrosive primercomposition. This is particularly advantageous since a full, strippable,coating system can be formed from only two coating layers.Alternatively, the undercoat composition can be applied as anintermediate layer over a cured anticorrosive primer composition beforecurable top coat composition is applied.

If the undercoat composition is an anticorrosive primer, it is usuallypreferred that the synthetic resin is an epoxy resin and the curingagent contains amine groups, for example the curing agent may be anamino-functional polyamide and/or an araliphatic diamine or an alkylene,arylene or alkarylene diamine. The curing agent can alternativelycontain carboxylic acid groups, or alternatively the synthetic resin-maycontain carboxylic acid or amine groups with the curing agent containingepoxide groups. Alternatively, the primer can be a thermosettingpolyurethane primer in which the synthetic resin contains hydroxylgroups and the curing agent contains blocked or unblocked isocyanategroups or the synthetic resin contains blocked or unblocked isocyanategroups and the curing agent contains hydroxyl groups. The primer coatingcan be applied from solution or dispersion in an organic solvent or canbe a high solids or solventless composition or may be an aqueouscomposition such as a dispersion, emulsion or solution or may be awater-thinnable primer.

If the undercoat composition is an intermediate layer, it is preferablya curable coating composition which is cured before the top coat isapplied. Such a curable composition can for example be based on an epoxyresin with an amino-functional curing agent or on a thermosettingpolyurethane as described above. The intermediate layer is mostpreferably based on a curable resin system which resists strippingsolvent, so that it can be overcoated after stripping of the top coat.Alternatively, the intermediate layer can be based on a thermoplasticresin, for example a thermoplastic polyester, polyurethane,fluoropolymer, acrylic polymer or polyamide. The intermediate layer canbe a high-solids or solventless composition or can be applied fromsolution or dispersion in water and/or an organic solvent, for example athermoplastic resin-based coating can be applied from aqueous dispersionor from a dispersion in water and organic solvent. A water-containingdispersion may for example contain a compound which facilitates theformation of the dispersion, such as a surfactant, which is volatile oris reactive with a component of the coating .composition, so that itdoes not remain in the coating film as a low molecular weight compoundbut evaporates from it or reacts with it on curing. A nitroparaffin, forexample, can be used as a volatile surfactant.

The acidic aromatic. polar organic compound is preferably anon-polymeric organic compound having a molecular weight in the range120 to 1500. The acidic aromatic compound generally contains at leastone acidic group such as a carboxylic acid group or sulphonic acid orphosphonic acid group present in free acid form or in salt form (inwhich case the compound may no longer be acidic). The salt may be an.amine salt of a primary, secondary or tertiary amine or an alkali metalor ammonium salt. The acidic group is preferably attached directly to anaromatic ring. Many of the effective compounds additionally contain atleast one hydroxy group and/or amino group. The acidic aromatic compoundpreferably has a molecular weight in the range 120 or 150 up to about1000 or 1500. Many of the effective compounds are known as dyes,although one of the most preferred compounds is gallic acid(3,4,5-trihydroxy-benzoic acid), which is not generally regarded as adye. Amine salts of gallic acid are particularly preferred.2,5-Dihydroxybenzoic acid is also effective, although3,5-dihydroxybenzoic acid is much less effective in giving stripping. 1,2-Dihydroxybenzene-3, 5-disulphonic acid is also effective in givingselective stripping. Examples of acidic dyes which have been foundeffective in giving selective stripping are Catechol Violet, Fast Green,Lissamine Green SF, Orange G, Amido Black, Methyl orange, sulphonylporphyrins, for example 5, 10, 15, 20-tetrakis(4-sulphonatophenyl)-porphine manganese (III) chloride of molecularweight 1023, bromocresol purple, calcion (a sulphonate-functional dye),fluorescent brightener 28 (an optical brightener), Nigrosin (Acid Black2), Brilliant Black BN, citrazinic acid, xylenol blue Na salt, ReactiveBlue 2, xylenol orange Na salt, Brilliant Blue R, Brilliant Blue Gand-chromoxane cyanine R. The chemical structures of many of these dyesare shown in WO-A-98/13148 . Aurin (rosolic acid), which is an. acid dyealthough it does not contain any carboxylic or stronger acid groups, isalso moderately effective. Mixtures of acidic aromatic materials can beused, for example mixtures of gallic acid with a dye such as Fast Green.Other examples of acid-functional acidic aromatic polar materials whichhave given selective stripping are a hyperbranchedpoly(5-hydroxyisophthalic acid) and poly(styrene sulphonate).

The acidic aromatic compound can be used in free acid. form or can beadded as a preformed salt, or a salt of the acidic aromatic compound canbe formed in situ in the composition; for example an amine salt can beformed by adding the amine and the acidic aromatic compound to thecoating composition.

where each R represents methyl and X⁻ is chloride, which does notcontain any acidic groups, has also been found to be effective and itsuse is also covered by the present invention, although other dyescontaining amine or hydroxyl groups with no acidic groups, such asmalachite green, night blue and crystal violet, have been found to beineffective. We believe that analogous compounds to Safranine O in whichR is hydrogen or another lower (1-4c) alkyl group and/or X⁻ is anotheranion are also effective.

The acidic aromatic compound is preferably present at a concentration ofat least 0.01% by weight, most preferably at least 0.05% or 0.1% byweight, in the coating composition as applied and preferably at least0.05% by weight, most preferably at least 0.1% or 0.2%. by weight, inthe dry coating to give the desired ease of removal of the top coat bythe stripping composition. A level of 0.2% to 0.5% by weight in the wetcoating composition, for example 0.25% by weight, is particularlypreferred. The level of acidic aromatic compound is preferably below 4%by weight, more preferably below 2% and most preferably below 1% byweight, of the wet coating composition (below 10%, most preferably below5 or 2%, by weight of the dry coating) for an anticorrosive primercomposition. Higher levels have also been found to provide strippabilityof the top coat, but care must be taken not to adversely affect theother properties of the primer.

An undercoat composition according to the invention which is ananticorrosive primer. generally contains an anticorrosive pigment suchas a chromate, phosphate, phosphonate or molybdate or a pigment asdescribed in WO 97/13888 and may contain one or more other pigmentsand/or fillers. Such an anticorrosive primer is generally applied at5-100 μm dry film thickness. The primer is generally stored as a 2- or3-pack composition, with the film-forming resin and the curing agentbeing stored in separate packages and in some cases catalyst and/oradditives being stored as a third package, all the packages to be mixedshortly before application of the coating to the substrate. The acidicaromatic compound can in general be included in any of the packs of sucha coating. In a 3-pack composition it is preferably included with thecatalyst and additives. In a 2-pack epoxy amine primer the acidicaromatic ;compound is preferably packaged with the amine, and in a2-pack polyurethane it is preferably packaged with thehydroxy-functional component.

If the undercoat composition is an interlayer applied over ananticorrosive primer, the level of acidic aromatic compound is generallybelow 5% by weight of the wet coating composition and below 10% byweight of the dry coating, and it is preferably below 1 to 2% by weightof the wet coating. The interlayer coating can be formulated as anadditional anticorrosive primer layer containing anticorrosive pigment,or it can be formulated similarly to a top coat with opacifying pigment,or it can simply contain a film-forming resin and the acidic aromaticcompound, optionally with one or more fillers or inert pigments. Theinterlayer coating can for example be applied at 1 to 100 μm, preferably5 to 25 μm, dry film thickness.

The top coat paint which is applied over an undercoat composition of theinvention is generally a curable coating comprising at least twoco-reactive components, which are usually packaged separately and mixedat application (for example by a twin-feed spray) or shortly beforeapplication (for example up to 8 hours before). The top coat paint,particularly for aircraft coatings, is preferably a polyurethane such asa polyester-urethane, although other. curable paints such as polyester,fluoropolymer or acrylic polymer paints, including reactive acrylicpolymer systems based on oxazolidine functionality, may be used. The topcoat paint is preferably applied from organic solvent solution,including high solids formulations, but it can in general be appliedfrom dispersion in water and/or organic solvent or as a solventless(100% solids) composition.

According to another aspect of the invention, a top coat compositioncomprises a film-forming synthetic resin having functional groups and acuring agent for crosslinking the synthetic resin and is characterisedin that the top coat composition contains a non-volatile acidic aromaticpolar organic compound in free acid or salt form, which compoundfacilitates the removal of the cured top coat by a stripping compositionfrom a cured undercoat of similar composition which does not contain thesaid acidic aromatic polar organic compound.

A process for overcoating a substrate having a cured primer coatingwith. a curable top coat composition which when cured is swellable by astripping composition is characterised according to this aspect of theinvention in that a first layer of the top coat composition is appliedto the primed substrate and cured and a second layer of the top coatcomposition containing a non-volatile acid aromatic polar organiccompound in free acid or salt form is coated over the first layer andcured, whereby the subsequent removal of the second layer from the firstlayer by the stripping composition is facilitated.

In some cases it is desirable that the top coat is strippable from anintermediate layer applied over an anticorrosive primer, for example arelatively temporary decorative coating applied over a permanent coatinglayer. We have found for example that a top coat composition accordingto the invention containing an acidic aromatic compound in a curedpolyurethane top coat can be stripped from an intermediate layer (firsttop coat layer) of similar composition. The top coat composition can forexample be based on a synthetic resin containing hydroxyl groups and acuring agent containing blocked or unblocked isocyanate groups, or on asynthetic resin containing blocked or unblocked isocyanate groups and acuring agent containing hydroxyl groups. The top coat composition canalternatively be based on a polyester, fluoropolymer or acrylic polymer,for example an oxazolidine-functional acrylic polymer. Both layers ofthe top coat composition are in general pigmented with one or moreopacifying pigments, for example titanium dioxide or a colour pigmentsuch as phthalocyanine.

The coating composition of the invention, whether it is an undercoat ora top coat, can in general be based on other curing chemistries. Forexample, the synthetic resin may contain activated —CH— groups bonded toat least two electron-withdrawing groups with a curing agent containingblocked or unblocked amine groups, epoxide groups or acrylic ormethacrylic groups, or the synthetic resin may contain acrylic ormethacrylic groups, epoxide groups or blocked or unblocked amine groupswith a curing agent containing activated —CH— groups. The preferredactivated —CH— groups are acetoacetate groups, although malonate ester,1,3-diketone or cyanoacetate groups are alternatives. For example, asynthetic resin containing acetoacetate groups can be used with a curingagent containing blocked or unblocked amine groups. An epoxy resin canbe used with a curing agent containing acetoacetate groups. A syntheticresin containing acrylic or methacrylic groups, for example a urethaneacrylate, can be used with a curing agent containing acetoacetategroups. In an alternative curing chemistry, the synthetic resin containsanhydride groups and the curing agent contains hydroxyl groups or thesynthetic resin contains hydroxyl groups and the-curing agent containsanhydride groups.

The acidic aromatic compound used in the top coat composition of theinvention can in general be any of those listed above for use in anundercoat, and it can be used in free acid or salt, for example aminesalt, form. The acidic aromatic compound is preferably present in thetop coat paint at a concentration of at least 0.01% by weight, morepreferably at least 0.05t and most preferably at least 0.1% by weight,of the coating composition (at least 0.05%, preferably at least 0.2%, byweight of the dry coating) up to 2% or 4% by weight of the coatingcomposition (4 or 10% by weight of the dry coating).

The top coat composition of the invention is usually a 2-pack or 3-packcomposition, and the acidic aromatic compound can be packaged in any ofthe packs as described above in connection with primers. The coatingcomposition of the invention, whether an undercoat or top coat, canalternatively be a single pack composition cured by moisture, oxygen,heat (for example a powder coating), UV, electron beam or laserradiation or by application of curing agent vapour. An undercoatcomposition according to the invention can be applied by extrusioncoating or as a preformed film.

After the top coat has been allowed to cure, preferably for at least 3days, it adheres firmly to the primer, and the coating system isresistant to spraying with salt-laden lo water and to immersion in“Skydrol” phosphate ester and generally to immersion in water, so thatneither separation of the top coat from the primer or undercoat norsignificant blistering occurs. However, when the top coat is treatedwith a paint stripper in which it swells, particularly a stripper of atype used commercially to strip aircraft paints, swelling of the topcoat is followed by detachment from the primer or undercoat. The topcoat lifts off the primer and may fall right away or is readily removedby low-pressure water washing. The top coat can readily be removedwithin a time which is generally 10 minutes to 5 hours and usually 0.5to 2 hours after application of the paint stripper, without any removalof the primer or of a cured interlayer undercoat. The primer or curedundercoat may be somewhat swollen or discoloured by the paint stripper,but it resists removal with the top coat and continues to adhere to thesubstrate. Any swelling of the primer or undercoat subsides as the paintstripper dries. The stripper most widely used to strip aircraft paints,particularly polyurethane top coats, is an aqueous thickened mixture ofbenzyl alcohol and formic acid. Alternative strippers are alkalinethickened aqueous benzyl alcohol or a stripper based on methylenechloride and phenol (used to strip certain fluoropolymer and acrylic topcoats).

After stripping of the top coat, the primer/undercoat can be recoatedwith a fresh top coat. In particular, an undercoat composition accordingto the invention can be re-coated with a fresh top coat and, after usein service, the fresh top coat can in its turn be readily stripped fromthe undercoat by a stripping solvent, allowing further repainting withtop coat.

The invention is illustrated by the following Examples.

EXAMPLE 1

Aluminium panels were chromic acid pickled and sprayed with 15-20 μm ofa high-solids (volatile organic content (VOC) 360 g/l) primer based on aliquid bisphenol F epoxy resin containing anticorrosive pigments and anamine curing agent, applied from organic solvent solution, and allowedto cure at ambient temperature. At the time of mixing of the epoxyprimer base and curing agent, a solution of gallic acid dissolved inacetone was added with stirring, so that the final concentration ofgallic acid in the mixed wet paint was 0. 2% by weight.

18 hours after the application of the primer with gallic acidincorporated, the primed panels were topcoated with 40-50 μm dry filmthickness of a pigmented polyurethane topcoat applied from an organicsolvent blend. The topcoat was a 2-pack polyurethane of a type commonlyused for aircraft, comprising a hydroxy-functional polyester componentand an aliphatic polyisocyanate component. The topcoat was allowed tocure under ambient conditions for one week before being tested forselective strippability and chemical resistance.

Some of the painted panels were coated with “Turco. 9090” formicacid-based stripping formulation, Turco 1270/5 benzyl alcohol-basedbasic stripper and Dasic D83 methylene. chloride-based stripper. After5-60 minutes the topcoat had lifted off the primer in each case.

The topcoat was then readily removed with a low-pressure water wash usedto clean off the stripping formulation, leaving a clean intact primersurface.

Some of the panels were tested to AIMS 04-04-014, including 14-days'immersion in Skydrol at 70° C. and 14 days' water immersion, and werefound to meet the requirements.

EXAMPLE 2

The process of Example 1 was repeated with a high-solids, bisphenolA-based epoxy primer containing anticorrosive pigments reacted with anamine curing agent, applied from organic solution. Gallic acid dissolvedin acetone was added at 0.2% by weight gallic acid based on the primer.The primer was overcoated after 20 hours with 60 μm of the topcoatapplied in Example 1. The panels were air dried for 7 days.

Some of the coated panels were coated with “Turco 9090”, Turco 1270/5and Dasic D83 stripping formulations. The topcoat lifted off the primerin each case and was then readily removed with a low-pressure waterwash, leaving a clean intact primer surface.

Some of the panels were tested to AIMS 04-04-014, including 14 days'immersion in Skydrol at 70° C. and 14 days' water immersion, and werefound to meet the requirements.

EXAMPLE 3

Chromic acid-pickled aluminium panels were coated with 12-18 μm of aconventional solids (VOC 550 g/l) primer based on a solid bisphenol Aepoxy resin containing anticorrosive pigment, cured with an amine andapplied from organic solvent. Just after the time of mixing the epoxyand curing agent, Tiron (Trade Mark) (which is the disodium salt of anaromatic sulphonic acid believed to be1,2-dihydroxybenzene-3,5-disulphonic acid), dissolved in deionisedwater, was added dropwise to give a final concentration of Tiron in themixed wet paint of 0.5% by weight. The primer was overcoated after 6hours with 45-60 μm of the topcoat applied in Example 1. The panels wereair dried for 7 days.

Some of the painted panels were coated with “Turco 90901” formicacid-based stripping formulation. After 30-60 minutes the topcoat hadlifted off the primer.

Some of the panels were coated with “Turco 5351” methylenechloride/phenol-based stripping formulation.

After 3 minutes the topcoat had lifted from all of these panels.

The topcoat was then readily removed with a low-pressure water wash usedto clean off the stripping solvent, leaving a clean intact primersurface.

Some of the panels were tested by immersion in hot (70°C.) Skydrolhydraulic fluid for 7 days and by 14 days' water immersion. Nodeterioration was observed.

EXAMPLE 4

Chromic acid-pickled aluminium panels were sprayed with 13-19 μm of aconventional solids epoxy primer (VOC 550 g/l) containing anticorrosivepigment, cured with an amine and applied from organic solvent.

This was allowed to air dry 20 for hours.

A solution of gallic acid was neutralised to pH6 with ammonia solutionand added dropwise with stirring to the epoxy component of a pigmentedpaint formulation based on the epoxy resin of Example 1, containing noanticorrosive pigments, cured with an amine curing agent, and appliedfrom organic solvent solution. The concentration of gallic acid ammoniumsalt in the mixed wet paint was equal to 0.2% by weight gallic acid.

The paint containing the amine salt of gallic acid described above wassprayed onto the anticorrosive epoxy primer with a film thickness of15-18 μm. The intermediate coat was allowed to dry for 5 hours atambient conditions. 40-60 μm of the topcoat as used in Example 1 wassprayed on top.

The panels were air dried for 3 days and force dried at 70° C. for 3hours.

Some of the painted panels were coated with “Turco 909011” formicacid-based stripping formulation. After.40-60 minutes the topcoat hadlifted off the primer.

Some of the panels were tested by immersion in hot (70° C.) Skydrolhydraulic fluid for 7 days. No deterioration was observed.

EXAMPLE 5

Chromic acid pickled aluminium panels were sprayed with 15-20 μm of awater-based epoxy primer (VOC<250 g/l) containing anticorrosive pigmentsreacted with an amine curing agent. Gallic acid neutralised to pH6 withammonia solution was added to the primer at 0.3% by weight. After 16hours' drying, the primer was overcoated with a high-solids 2-componentpolyurethane topcoat at dry film thickness of 60 μm. The topcoat wasallowed to cure at ambient conditions for one week before being testedfor selective strippability and chemical resistance.

Panels were coated with Turco 9090 and Turco 1270/5 and gave selectivestripping after 45-60 minutes.

Panels were found to be resistant to immersion in Skydrol at 70° C. for7 days and in water at 23° C. for 7 days.

EXAMPLE 6

Chromic acid pickled aluminium panels were coated with 15-20 μm ofeither a high-solids epoxy primer as in Example 1 or a conventionalsolids epoxy primer as in Example 3, containing anticorrosive pigments,reacted with an amine curing agent, applied from organic solventsolution and allowed to cure at ambient temperature.

The epoxy primer was allowed to dry for 4 hours before overcoating withan intermediate coating formulation containing the triethanolamine saltof gallic acid. The intermediate formulation contained a mixture of anaqueous polyurethane dispersion and an aqueous acrylic dispersion intowhich was added 2 g (per 100 g of polymer dispersion of pre-manufacturedtriethanolamine salt of gallic acid in water. The intermediateformulation also contained rheology modifiers and flow additives and hada VOC of 370 g/l. The intermediate formulation was applied at 20 μm dryfilm thickness.

After 4 hours' cure at ambient temperature the intermediate wasovercoated with 60 μm of a high-solids pigmented polyurethane topcoatapplied from an organic solvent blend.

The scheme was allowed to cure at ambient temperature for one weekbefore being tested for selective strippability and chemical resistance.

Some of the panels were coated with “Turco 1270/5” benzyl alcohol-basedstripping formulation. After 25 minutes at 25° C. the topcoat andintermediate had lifted off the primer. The intermediate was wiped offwith a cleaning solution, leaving the primer surface intact.

Some of the panels were tested by immersion in hot (70° C.) Skydrolhydraulic fluid for 7 days, and some were immersed in water at 40° C.for 7 days. In both cases no deterioration was observed.

1. An intermediate layer composition for applying over a cured primer coated substrate, which intermediate layer composition comprises a film-forming synthetic resin, characterized in that the intermediate layer composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the intermediate layer composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured intermediate layer composition, and characterized in that the synthetic resin has functional groups whereby it is curable and the intermediate layer composition contains a curing agent for crosslinking the synthetic resin, wherein the synthetic resin and curing agent are characterized by at least one of the following: the synthetic resin is an epoxy resin and the curing agent contains carboxylic acid groups; the synthetic resin contains amine or carboxylic acid groups and the curing agent contains epoxide groups; the synthetic resin contains hydroxyl groups and the curing agent contains blocked or unblocked isocyanate groups; the synthetic resin contains blocked or unblocked isocyanate groups and the curing agent contains hydroxyl groups; the synthetic resin contains activated —CH— groups bonded to at least two electron withdrawing groups and the curing agent contains blocked or unblocked amine groups, epoxide groups or, acrylic or methacrylic groups; the synthetic resin contains blocked or unblocked amine groups, epoxide groups or, acrylic or methacrylic groups and the synthetic resin contains activated —CH— groups bonded to at least two electron withdrawing groups; the synthetic resin contains anhydride groups and the curing agent contains hydroxyl groups; and the synthetic resin hydroxyl groups and the curing agent contains anhydride groups.
 2. An intermediate layer composition for applying over a cured primer coated substrate, which intermediate layer composition comprises a film-forming synthetic resin, characterized in that the intermediate layer composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the intermediate layer composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured intermediate layer composition, and characterized in that the synthetic resin has functional groups whereby it is curable and the intermediate layer composition contains a curing agent for crosslinking the synthetic resin, and characterized in that the synthetic resin is an epoxy resin and the curing agent contains carboxylic acid groups, or the synthetic resin contains amine or carboxylic acid groups and the curing agent contains epoxide groups.
 3. An intermediate layer composition for applying over a cured primer coated substrate, which intermediate layer composition comprises a film-forming synthetic resin, characterized in that the intermediate layer composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the intermediate layer composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured intermediate layer composition, and characterized in that the synthetic resin has functional groups whereby it is curable and the intermediate layer composition contains a curing agent for crosslinking the synthetic resin, and characterized in that the synthetic resin contains hydroxyl groups and the curing agent contains blocked or unblocked isocyanate groups, or the synthetic resin contains blocked or unblocked isocyanate groups and the curing agent contains hydroxyl groups.
 4. An intermediate layer composition for applying over a cured primer coated substrate, which intermediate layer composition comprises a film-forming synthetic resin, characterized in that the intermediate layer composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the intermediate layer composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured intermediate layer composition, and characterized in that the synthetic resin has functional groups whereby it is curable and the intermediate layer composition contains a curing agent for crosslinking the synthetic resin, and characterized in that the synthetic resin contains activated —CH— groups bonded to at least two electron-withdrawing groups and the curing agent contains blocked or unblocked amine groups, epoxide groups or acrylic or methacrylic groups, or the synthetic resin contains acrylic or methacrylic groups, epoxide groups or blocked or unblocked amine groups and the curing agent contains activated —CH— groups bonded to at least two electron-withdrawing groups.
 5. An intermediate layer composition for applying over a cured primer coated substrate, which intermediate layer composition comprises a film-forming synthetic resin, characterized in that the intermediate layer composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the intermediate layer composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured intermediate layer composition, and characterized in that the synthetic resin has functional groups whereby it is curable and the intermediate layer composition contains a curing agent for crosslinking the synthetic resin, and characterized in that the synthetic resin contains anhydride groups and the curing agent contains hydroxyl groups, or the synthetic resin contains hydroxyl groups and the curing agent contains anhydride groups.
 6. An intermediate layer composition according to claim 1, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 7. An intermediate layer composition according to claim 1, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 8. An intermediate layer composition according to claim 1, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 9. An intermediate layer composition according to claim 7, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 10. An intermediate layer composition according to claim 1, characterized in that the aromatic compound is gallic acid.
 11. An intermediate layer composition according to claim 1, characterized in that the non-volatile acidic aromatic polar organic compound is present therein at 0.05 to 1% by weight.
 12. A top coat composition for overcoating a cured coating, which top coat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the top coat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the top coat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound comprises at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured composition, and wherein the synthetic resin and the curing agent are characterized by at least one of the following: the synthetic resin contains hydroxyl groups and the curing agent contains blocked or unblocked isocyanate groups; the synthetic resin contains blocked or unblocked isocyanate groups and the curing agent contains hydroxyl groups; the synthetic resin contains activated —CH— groups bonded to at least two electron withdrawing groups and the curing agent contains blocked or unblocked amine groups, epoxide groups or, acrylic or methacrylic groups; the synthetic resin contains blocked or unblocked amine groups, epoxide groups or, acrylic or methacrylic groups and the synthetic resin contains activated —CH— groups bonded to at least two electron withdrawing groups; the synthetic resin contains anhydride groups and the curing agent contains hydroxyl groups; the synthetic resin contains hydroxyl groups and the curing agent contains anhydride groups; and the synthetic resin is a fluoropolymer.
 13. A top coat composition for overcoating a cured coating, which top coat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the top coat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the top coat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound comprises at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured composition, and characterized in that the synthetic resin is a polyester, fluoropolymer or acrylic polymer.
 14. A top coat composition for overcoating a cured coating, which top coat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the top coat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the top coat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound comprises at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured composition, and characterized in that the synthetic resin contains hydroxyl groups and the curing agent contains blocked or unblocked isocyanate groups, or the synthetic resin contains blocked or unblocked isocyanate groups and the curing agent contains hydroxyl groups.
 15. A top coat composition for overcoating a cured coating, which top coat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the top coat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the top coat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound comprises at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured composition, and characterized in that the synthetic resin contains activated —CH— groups bonded to at least two electron-withdrawing groups and the curing agent contains blocked or unblocked amine groups, epoxide groups or acrylic or methacrylic groups, or the synthetic resin contains acrylic or methacrylic groups, epoxide groups or blocked or unblocked amine groups and the curing agent contains activated —CH— groups bonded to at least two electron-withdrawing groups.
 16. A top coat composition for overcoating a cured coating, which top coat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the top coat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the top coat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured, and with the proviso that when the non-volatile acidic aromatic polar organic compound comprises at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured composition, and characterized in that the synthetic resin contains anhydride groups and the curing agent contains hydroxyl groups, or the synthetic resin contains hydroxyl groups and the curing agent contains anhydride groups.
 17. A top coat composition according to claim 12, characterized in that the non-volatile acidic aromatic polar organic aromatic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 18. A top coat composition according to claim 12, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 19. A top coat composition according to claim 12, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 20. A top coat composition according to claim 18, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 21. A top coat composition according to claim 12, characterized in that the non-volatile acidic aromatic polar organic compound is gallic acid.
 22. A top coat composition according to claim 12, characterized in that the non-volatile acidic aromatic polar organic compound is present therein at 0.05 to 1% by weight.
 23. A process for coating a substrate comprising applying to the substrate a curable undercoat composition and curing the undercoat, and applying a curable top coat composition over the undercoat and curing the top coat, the cured top coat being swellable by a stripping composition, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal of the top coat from the undercoat by the stripping composition; and characterized in that the undercoat composition comprises a film-forming synthetic resin characterized in that the synthetic resin has functional groups whereby it is curable and the undercoat composition contains a curing agent for crosslinking the synthetic resin, wherein the synthetic resin and curing agent are characterized by at least one of the following: the synthetic resin contains an epoxy resin and the curing agent contains carboxylic acid groups; the synthetic resin contains amine or carboxylic acid groups and the curing agent contains epoxide groups, the synthetic resin contains hydroxyl groups and the curing agent contains blocked or unblocked isocyanate groups; the synthetic resin contains blocked or unblocked isocyanate groups and the curing agent contains hydroxyl groups; the synthetic resin contains activated —CH— groups bonded to at least two electron withdrawing groups and the curing agent contains blocked or unblocked amine groups, epoxide groups or, acrylic or methacrylic groups; the synthetic resin contains blocked or unblocked amine groups, epoxide groups or, acrylic or methacrylic groups and the curing agent contains activated —CH— groups bonded to at least two electron withdrawing groups; the synthetic resin contains anhydride groups and the curing agent contains hydroxyl groups; and the synthetic resin contains hydroxyl groups and the curing agent contains anhydride groups.
 24. A process according to claim 23, characterized in that the undercoat composition is an anticorrosive primer composition.
 25. A process for coating a substrate comprising applying to the substrate a curable undercoat composition and curing the undercoat, and applying a curable top coat composition over the undercoat and curing the top coat, the cured top coat being swellable by a stripping composition, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal of the top coat from the undercoat by the stripping composition, and characterized in that the non-volatile acidic aromatic polar organic compound is present at 0.05 to 1% by weight in the primer composition.
 26. A process for coating a substrate comprising applying to the substrate a curable undercoat composition and curing the undercoat, and applying a curable top coat composition over the undercoat and curing the top coat, the cured top coat being swellable by a stripping composition, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal of the top coat from the undercoat by the stripping composition, and characterized in that the undercoat composition is applied as an intermediate layer over a cured anticorrosive primer composition before the curable top coat composition is applied.
 27. A process according to claim 23, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 28. A process according to claim 23, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 29. A process for coating a substrate comprising applying to the substrate a curable undercoat composition and curing the undercoat, and applying a curable top coat composition over the undercoat and curing the top coat, the cured top coat being swellable by a stripping composition, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal of the top coat from the undercoat by the stripping composition, and characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 30. A process according to claim 28, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 31. A process for coating a substrate comprising applying to the substrate a curable undercoat composition and curing the undercoat, and applying a curable top coat composition over the undercoat and curing the top coat, the cured top coat being swellable by a stripping composition, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal of the top coat from the undercoat by the stripping composition, and characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group and additionally contains at least one hydroxy group and/or amino group.
 32. A process according to claim 23, characterized in that the non-volatile acidic aromatic polar organic compound is gallic acid.
 33. A process for coating a substrate comprising applying to the substrate a curable undercoat composition and curing the undercoat, and applying a curable top coat composition over the undercoat and curing the top coat, the cured top coat being swellable by a stripping composition, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the removal of the top coat from the undercoat by the stripping composition, and characterized in that the non-volatile acidic aromatic polar organic compound is present in amine salt form.
 34. A process for overcoating a substrate having a cured primer coating to facilitate the removal by a stripping composition of a subsequently applied top coat, characterized in that a coating composition comprising a film-forming resin and a non-volatile acidic aromatic polar organic compound in free acid or salt form, which compound facilitates the said removal of the top coat, is coated over the cured primer coating before the top coat is applied.
 35. A process according to claim 34, characterized in that the said coating composition is a curable coating composition and is cured before the top coat is applied.
 36. A processing according to claim 34, characterized in that the said coating composition comprises a thermoplastic synthetic resin.
 37. A process according to claim 34, characterized in that the said coating composition is applied from an aqueous dispersion.
 38. A process according to claim 37, characterized in that the aqueous dispersion contains a volatile or reactive surfactant which facilitates the formation of the dispersion and which evaporates from the coating on application or reacts with the coating composition on curing.
 39. A process according claim 34, characterized in that the non-volatile acidic aromatic polar organic compound is present at 0.05 to 5% by weight in the said coating composition.
 40. A process according to claim 34, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 41. A process according to claim 34, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 42. A process according to claim 34, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 43. A process according to claim 40, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 44. A process according to claim 42, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 45. A process according to claim 34, characterized in that the non-volatile acidic aromatic polar organic compound is gallic acid.
 46. A process according to claim 34, characterized in that the non-volatile acidic aromatic polar organic compound is present in amine salt form.
 47. A process for overcoating a substrate having a cured primer coating with a curable top coat composition which when cured is swellable by a stripping composition, characterized in that a first layer of the top coat composition is applied to the primed substrate and cured, and a second layer of the top coat composition containing non-volatile acidic aromatic polar organic compound in free acid or salt form is coated over the first layer and cured, whereby the subsequent removal of the second layer from the first layer by the stripping composition is facilitated.
 48. A process according to claim 47, characterized in that the non-volatile acidic aromatic polar organic compound is present at 0.05 to 1% by weight in the top coat composition applied as the second layer.
 49. A process according to claim 47, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 50. A process according to claim 47, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 51. A process according to claim 47, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 52. A process according to claim 40, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 53. A process according to claim 51, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 54. A process according to claim 47, characterized in that the non-volatile acidic aromatic polar organic compound is gallic acid.
 55. A process according to claim 47, characterized in that the non-volatile acidic aromatic polar organic compound is present in amine salt form.
 56. A process for coating a substrate comprising applying to the substrate an undercoat composition, applying a curable top coat composition over the undercoat and curing the top coat, the cured top coat being swellable by a stripping composition, characterized in that the undercoat composition contains a compound of the structure

where each R represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms and X represents an anion, which compound facilitates the removal of the top coat from the undercoat by the stripping composition.
 57. An undercoat composition for coating a substrate, which undercoat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group and at least one group selected from a hydroxy group and an amino group.
 58. An undercoat composition for coating a substrate, which undercoat composition comprises a film-forming synthetic resin, characterized in that the undercoat composition contains a non-volatile acidic aromatic polar organic compound in amine salt form.
 59. A top coat composition for applying as the outer coating on a substrate, which top coat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the top coat composition contains a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group and at least one group selected from a hydroxy group and an amino group.
 60. A top coat composition for applying as the outer coating on a substrate, which top coat composition comprises a film-forming synthetic resin having functional groups and a curing agent for crosslinking the synthetic resin, characterized in that the top coat composition contains a non-volatile acidic aromatic polar organic compound in amine salt form.
 61. An undercoat composition comprising: a curable film-forming synthetic resin containing amino or carboxylic acid groups; a curing agent containing epoxide groups for cross-linking the film-forming synthetic resin; and a non-volatile acidic aromatic polar organic compound in free acid or salt form, wherein the non-volatile acidic aromatic polar organic compound comprises at least one carboxylic acid group and at least one amino group; wherein the undercoat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured; and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured undercoat composition.
 62. An undercoat composition according to claim 61, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 63. An undercoat composition according to claim 61, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 64. An undercoat composition according to claim 61, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 65. An undercoat composition according to claim 63, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 66. An undercoat composition according to claim 61, characterized in that the aromatic compound is gallic acid.
 67. An undercoat composition according to claim 61, characterized in that the non-volatile acidic aromatic polar organic compound is present therein at 0.05 to 1% by weight.
 68. An undercoat composition comprising: a curable film-forming synthetic resin; a curing agent for cross-linking the film-forming synthetic resin; and a non-volatile acidic aromatic polar organic compound in free acid or salt form; wherein the film-forming synthetic resin contains hydroxyl groups and the curing agent contains blocked or unblocked isocyanate groups, or the film-forming synthetic resin contains blocked or unblocked isocyanate groups and the curing agent contains hydroxyl groups; and wherein the undercoat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured; and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured undercoat composition.
 69. An undercoat composition according to claim 68, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 70. An undercoat composition according to claim 68, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 71. An undercoat composition according to claim 68, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 72. An undercoat composition according to claim 70, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 73. An undercoat composition according to claim 68, characterized in that the aromatic compound is gallic acid.
 74. An undercoat composition according to claim 68, characterized in that the non-volatile acidic aromatic polar organic compound is present therein at 0.05 to 1% by weight.
 75. An undercoat composition comprising: a curable film-forming synthetic resin containing acrylic or methacrylic groups, epoxide groups or blocked or unblocked amine groups; a curing agent containing activated —CH— groups bonded to at least two electron-withdrawing groups for cross-linking the film-forming synthetic resin; and a non-volatile acidic aromatic polar organic compound in free acid or salt form; wherein the undercoat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured; and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured undercoat composition.
 76. An undercoat composition according to claim 75, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 77. An undercoat composition according to claim 75, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 78. An undercoat composition according to claim 75, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 79. An undercoat composition according to claim 77, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 80. An undercoat composition according to claim 75, characterized in that the aromatic compound is gallic acid.
 81. An undercoat composition according to claim 75, characterized in that the non-volatile acidic aromatic polar organic compound is present therein at 0.05 to 1% by weight.
 82. An undercoat composition comprising: a curable film-forming synthetic resin; a curing agent for crosslinking the film-forming synthetic resin; and a non-volatile acidic aromatic polar organic compound in free acid or salt form; wherein the film-forming synthetic resin contains anhydride groups and the curing agent contains hydroxyl groups, or the synthetic resin contains hydroxyl groups and the curing agent contains anhydride groups; and wherein the undercoat composition comprises the non-volatile acidic aromatic polar organic compound in free acid or salt form when cured; and with the proviso that when the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group, the compound containing at least one sulfonic acid group is present in an amount greater than 0.05% by weight of the total weight of the cured undercoat composition.
 83. An undercoat composition according to claim 82, characterized in that the non-volatile acidic aromatic polar organic compound is a non-polymeric organic compound having a molecular weight in the range of 120 to
 1500. 84. An undercoat composition according to claim 82, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one carboxylic acid group.
 85. An undercoat composition according to claim 82, characterized in that the non-volatile acidic aromatic polar organic compound contains at least one sulfonic acid group.
 86. An undercoat composition according to claim 84, characterized in that the non-volatile acidic aromatic polar organic compound additionally contains at least one hydroxy group and/or amino group.
 87. An undercoat composition according to claim 82, characterized in that the aromatic compound is gallic acid.
 88. An undercoat composition according to claim 82, characterized in that the non-volatile acidic aromatic polar organic compound is present therein at 0.05 to 1% by weight. 